The Wars of the Damned.
This is the time that comes in the wake of the Coming of the Azure Flames that destroyed the Old World, ending with the rise of the Empire of Man. In addition to The Necromancer and The Archmage, other notable figures arose from the ashes in various parts of the world and became dominant in their regions. Because of the global reach of The Necromancer, most of these figures first went to war with The Necromancer in order to secure their base of power- always including a survivor population that rallied to that figure's banner due to their obvious power to oppose the Master of All Flesh.
That means that Solador and The Archmage are one example of many, and not all of them are human. These regional players, separated from one another geographically, are what kept The Necromancer in check enough to wear him down over time. However, they did not do this emergently; they had the covert aid of The Hidden City, providing intelligence and intervention as required when required. The Necromancer comes to recognize that he has a hidden enemy aiding his opponents rather swiftly, and even comes to know The Hidden City, but never touches it because he never figures out how to get to it. (Dude never saw Tron, and no one told him, so that idea never occurred to him.)
This period lasted a couple of centuries, with the tipping point being the Empire of Man destroying The Necromancer and his Empire of the Dead (at that time, with the aid of The Hidden City). The Empire would go on to conquer all of the remaining players, except The Hidden City, in turn until Mankind once more was uncontested master of Earth.
This entire period would last five centuries, from the cataclysm of the Azure Flames to the final conquest of The Empire of Man. As with the fall of Western Rome, the period of chaos and instability was actually rather small. The length stems from the conflict between the successor states that arose from the ashes, and once one party realized it was a kingmaker it played the field until it chose a king.